Add rendering for landuse=salt_pond

[jeisenbe]

Fixes #545
Follow-up to #3905

Changes proposed in this pull request:

• Add rendering of landuse=salt_pond areas with fill in ocean color (slightly lighter than water color, #b5d7e3 - Lch(84,13,227)
• Add pattern overlay for landuse=salt_pond from @imagico alt-colors style
• Add text label for landuse=salt_pond
  Test rendering with links to the example places:

https://www.openstreetmap.org/#map=15/41.5849/9.2909
Salt evaporation pond area in Corsica

Before

After

https://www.openstreetmap.org/#map=14/-34.8186/138.5977
Salt evaporation ponds (salterns) in South Australia

Before

After

[pnorman]

With a ΔE of 2.9 the two blues are not distinguishable. If the intent is to have blues you can tell the difference between, they need to be farther apart. If the intent is to have them look the same, they should be exactly the same.

[jeisenbe]

It's supposed to be a subtle difference. But since it makes sense in the context of changing the ocean-color too, I have removed that change and just used @water-color for salt_pond, for now.

changes made

[pnorman]

I would have liked to seen more reviews from people in areas where salt ponds are common, but I'm okay with merging this.

[imagico]

I think it looks fine.

Already with the original design i was a bit worried about the pattern being too strong - and it is even stronger with the darker background color. On the other hand darkening the symbols to weaken the pattern would make it less recognizable because the color association white -> salt would be lost.

[jeisenbe]

I think that the water color should evenually be changed to be the same as intermittent salt water lakes, and that should be a slightly lighter color, so the pattern will not look quite as strong then.